Hi,
Ive just finished halo 5 on heroic and havent had my achievements unlocked, ive reset the console restarted the game and redone the last mission and still nothing 
The second last mission didn’t register for you. The Breaking which is great fun :P.
I had this happen to me twice on mcc, redo that mission and it should work. As far as I’ve noticed it’s usually when I skip the cutscenes it has happened.
Go to the second to last mission and hit continue from last checkpoint. Make sure the game has saved fully once you complete the mission.